1. Field of the Invention
At least one embodiment of the invention relates to a permanently or temporarily implantable medical device having an electrical lead that extends longitudinally.
2. Description of the Related Art
Such devices, for example electrode leads for electrical stimulation, have the disadvantage that the electrical conductors thereof can heat up in an MRI machine, because the alternating magnetic fields in an MRI machine induce electrical currents in the electrical conductor that are not insignificant. For this reason, patients with heart pace makers usually cannot be examined in an MRI machine at the current time, or can only be examined in a limited manner.
Implantable heart pacemakers or defibrillators typically have at least one stimulation electrode lead attached to said pacemaker, wherein said electrode lead has a standardized electrical connection at its proximal end, said end being provided for connection to the heart pace maker or defibrillator, and wherein said electrode lead has one or multiple electrode poles on it distal end, said distal end being provided for actively locating the same in the heart. Such an electrode pole serves to release an electrical impulse, for instance to the (myocardial) tissue of the heart, or to sense electrical fields in order to be able to sense an activity, such as heart activity.
For this purpose, electrode poles typically form electrically conductive surface sections of an electrode lead. Electrode poles are typically provided in the form of a ring around the electrode lead, or in the form of a point or tip electrode at the distal end of the electrode lead.
The electrode poles are connected to contacts of the electrical connection of the electrode lead on the proximal ends thereof, in an electrically-conducting manner via one or multiple electrical conductors. Consequently, one or multiple electrical leads, which electrically connect one or multiple electrode poles to one or multiple contacts, run between the contacts of the electrical connection of the electrode leads at the proximal end thereof and the electrode poles at the distal end thereof. These electrical leads can be used to both transmit stimulation impulses to the electrode poles and also to transmit electrical signals obtained by means of the electrode poles to the proximal end of the electrode lead. In the following description, said electrical leads are also characterized as functional leads.
Such functional leads are electrical leads which are necessary for the functions of each electrode lead. As such, they are subject to the danger that electrical current can be induced in them by external alternating magnetic fields. This electrical current can, for instance, lead to an undesirable heating of the functional leads or of the electrode poles connected to the same, or can lead to a discharge of corresponding current via the electrode poles into the surrounding tissue, thereby heating the surrounding tissue.
The use of a diode to limit current in an electrode conductor is known from EP 2 067 501 A2. The solution has the advantage that it functions substantially independently of the interference frequency and the current induced by the same. However, the relatively high voltages to which a diode is exposed, particularly under high-performance conditions in a magnetic resonance imager with interfering alternating magnetic fields, the diode being switched into the circuit in the blocking direction, can lead to destruction of the diode. As a consequence, the diode ceases to conduct current. In a heart pace maker or a defibrillator, this leads to loss of the therapeutic or diagnostic function of an electrode pole, which becomes uncoupled from the control device due to the destroyed and therefore non-conducting diode.
The known solution mentioned above is also sensitive to mechanical stress. This is because semiconductors are very fragile by nature, and must be connected to metal contacts via a bond which is likewise mechanically susceptible.